Optimising LSP conditions and modelling the geometric effects on residual stress

Shapiro, Karen, Achintha, Mithila, Withers, Phillip and Nowell, David (2011) Optimising LSP conditions and modelling the geometric effects on residual stress. 3rd International Conference on Laser Peening, Osaka, Japan. 10 - 13 Oct 2011.

Abstract

compressive stresses close to the surface of a metal component. The method is particularly useful in the surface treatments of highly-stressed alloys used in the aerospace industry. LSP typically produces compressive zones over 1.5-2.0mm deep, in comparison to about 0.25mm produced by conventional shot peening. The laser parameters can be relatively easily controlled, allowing the process to be tailored to specific design requirements. Additionally, the flexibility of the process allows peening of complex geometries (e.g. leading edges of aero-engine blades). However, a comprehensive analytical or numerical method for predicting the residual stress (RS) distributions generated by LSP is lacking. Consequently, the method is not being exploited as effectively as it might be and in some situations (e.g. in complex geometries) the process has failed to give the expected benefits.

The current study forms part of a wider programme of work involving a number of industrial and academic collaborators and the study developed a comprehensive understanding of the LSP process through interpretation of experimental and model results. The experimental work involves measuring and understanding how laser process parameters, specimen geometry and material properties affect the RS fields caused by LSP. X-ray and neutron diffraction techniques have been used to measure RS profiles in Ti-6Al-4V and aluminium alloys (Al2024 and Al7050), all of which are widely used in the aerospace industry for a range of LSP parameters. The experimental results are used to determine the optimal peening conditions and also to quantify the fatigue performance of specimens with a wide range of geometries.

A more physically-based eigenstrain (i.e. misfit strain) model which considered the plastic strains introduced by the process has been developed to determine the RS field generated by LSP [1]. Due to propagation of the shock wave generated by a laser shock, the top layers of the specimen experience plastic deformation, and on relaxation the deformed material is loaded in compression by the undeformed material which surrounds this region. Thus, the plastic deformation caused by the shock wave generates the RS field, and also once the plastic deformations are fully stabilised the response of the workpiece is elastic. In the present model, the effect of the LSP pulse is first modelled as a dynamic pressure load in an explicit FE model in order to determine the stabilised plastic strain distribution, which is then incorporated into a static FE model as an eigenstrain. The elastic response of the static FE model gives the RS distribution generated by the original laser pulse.

The eigenstrain analysis has a number of advantages. Firstly, once the eigenstrains have been determined, the
complete RS distribution can be reconstructed through a single elastic analysis, and hence, the solution can be
determined at a manageable computational cost. The formulation of the solution this way ensures strain compatibility, global stress equilibrium, and matches the boundary conditions. The results have shown that the LSP process parameters can be directly linked to the underlying eigenstrain distribution, and also, a given laser setting produces similar eigenstrain distributions in workpieces (of a given material) of different geometries. Therefore, it is possible to undertake a rapid assessment of the RS field caused in new or complex geometries, and also, the effect of multiple LSP shots simply by installing the appropriate eigenstrain distributions at the correct locations within the component.

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Contributors

Author: Mithila Achintha

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